Environmental concerns have created a demand for apparatus and methods for treating a variety of waste. For instance, treatment of waste which contains constituents known to damage human health or ecosystems, such as hazardous metals, radioactive materials, hazardous organic compounds, or other hazardous materials are of particular concern because hazardous substances that may accumulate in ground water or in the air, pose a danger to human or animal life. In addition, some types of compounds, such as radioactive materials, remain in the environment in a hazardous condition for a relatively long time.
One particular approach for treatment and storage of such hazardous materials is “vitrification.” Hazardous materials may be vitrified when they are combined with glass forming materials and heated to relatively high temperatures. During vitrification, some of the hazardous constituents, such as hazardous organic compounds, may be destroyed by the high temperatures, or may be recovered as fuels. Other hazardous constituents, which are able to withstand the high temperatures, may form a molten state which then cools to form a stable vitrified glass. The vitrified glass may demonstrate relatively high stability against chemical and environmental attack as well as a relatively high resistance to leaching of the hazardous components contained therein.
One type of apparatus that has proven to be effective to vitrify waste materials is a cold-crucible-induction melter (CCIM). A cold-crucible-induction melter typically may comprise a water-cooled crucible disposed within an induction coil, or other inductor, usually formed along a spiral path surrounding therearound. Generally, an induction coil, carries varying electric currents that generate associated varying magnetic fields thereby inducing eddy currents within electrically conductive materials encountered thereby. The varying electromagnetic fields generated by the current within an inductor may be described as the “flux” thereof.
Waste may be induction heated directly if it is sufficiently electrically conductive and thereby vitrified. However, the waste and glass forming materials used in vitrification systems may be relatively non-electrically conductive at room temperatures. Therefore, an electrically conductive material may be used to initially indirectly heat at least a portion of the waste to a molten state, at which point the waste may become more electrically conductive so that when varying current is conducted through the induction coil, conductive molten waste may be induction heated by way of eddy currents generated therein. Of course, non-electrically-conductive waste materials nearby the electrically conductive molten waste, due to the heat generated therein, may be indirectly heated and thus, melted.
Alternatively, U.S. Pat. No. 6,476,285 to Kobayashi et al. discloses two induction power supplies connected to two induction coils disposed around a cold crucible for applying different frequencies of induction heating flux in order to heat materials having relatively high electrical resistance and materials having relatively low electrical resistance.
As a further advantage of cold-crucible-induction melter vitrification systems, molten glass within the water-cooled crucible may form a solid layer (skull layer), which inhibits or prevents direct contact of the high temperature molten glass with the interior surface of the crucible. Furthermore, because the crucible itself is cooled with water, in combination with the insulative properties of the skull layer, high-temperature melting may be achieved without being substantially limited by the heat-resistance or melting point of the crucible.
Another challenge that may be encountered in vitrifying waste relates to discharging the molten waste from the crucible. Conventional methods of discharging the molten material typically include approaches such as: 1) a system which allows the molten waste to pour from the crucible by tilting thereof; 2) a system which allows the molten waste to flow from a bottom drain disposed within the crucible by gravity or by pressurizing the inside of the crucible; 3) a system which allows the molten waste to flow from a side drain disposed within the crucible wall by gravity or by pressure control of the inside of the crucible; and 4) a system which allows the molten waste to flow from the melt pool via an underflow/overflow weir arrangement disposed within the crucible at the bottom and side of the melter by gravity or by pressurize control of the weir cavities. Pouring systems may not be favored in the case of hazardous waste, because spillage and exposure of the hot upper surface of the molten waste to the environment may generally create additional volatized contamination and safety issues. Side drains may not be favored due to interference with crucible heating devices, as well as the inability to completely evacuate the crucible upon deployment. Underflow/overflow weir arrangements are typically made from refractory materials and may not be favored due to their complexity, bulkiness, and maintenance issues.
Conventional bottom drains including heating systems and cooling systems have been employed in the past for a glass melting furnace used in vitrification of radioactive wastes. Conventional bottom drains may be typically located at the geometric center or aligned with the central axis of the crucible and may include a separate induction coil disposed therearound that may be selectively energized to effect discharge of the glass or waste from the crucible. Examples of conventional bottom drain assemblies including separate induction coils therearound may be found in U.S. Pat. No. 5,901,169 to Kobayashi and U.S. Pat. No. 6,219,372 to Zabata et al. To discharge the molten glass inside the crucible, the discharging nozzle is heated so as to melt the solidified glass inside the nozzle and allow it to flow down by gravity, and at the same time, the molten glass inside the furnace can be discharged. Discharge of molten glass may be prevented as glass within the bottom drain solidifies subsequent to heating thereof ceasing. Therefore, preventing discharge of molten glass from conventional bottom drain systems may rely upon the cooling dynamics of the surrounding environment and may not be sufficiently responsive. To address this issue, U.S. Pat. No. 4,460,398 to Sasaki includes cooling air that freezes the glass exiting the bottom drain assembly. Further, U.S. Pat. No. 6,307,875 to Tsuda et al. employs a relatively low frequency induction heating flux to effect a suspension of the molten glass exiting the bottom drain assembly by way of forces generated thereby.
Notwithstanding the prior art approaches to allowing and preventing discharge from a cold-crucible-induction melter, there exists a need for an improved bottom drain.
Yet another shortcoming of conventional cold-crucible-induction melter operation concerns the head assembly or lid assembly that is used therein. More particularly, although conventional head assemblies for cold-crucible-induction melters may be fabricated from refractory materials, often the outer temperature of the head assembly during operation is unacceptably high and poses safety hazards to those around the system. Also, often waste materials are able to infiltrate the refractory of the head assembly, and therefore become an additional waste problem. Another conventional approach for cooling the head assembly without including conventional refractory materials is to cool the head assembly by water flowing in a jacket therearound. Although water-cooling may sufficiently cool the head assembly, water-cooled head assemblies may also remove an unacceptable amount of heat from the cold-crucible-induction melter during operation, thus reducing the desired headspace plenum (zone between the hot melt pool and the head internal surface) temperature therein. As a further consideration, the thermal characteristics of the head assembly may affect the formation and maintenance of a solid layer of material formed on the upper surface of the molten waste. Such a layer may typically be termed a “cold cap” and may be beneficial to prevent volatized hazardous materials from escaping the crucible. Therefore, there exists a need for an improved cold-crucible-induction melter head assembly.
In addition, conventional cold-crucible-induction melters may lack the ability to easily control the thermal profile of the contents of the crucible. For instance, it may be desirable to preferentially heat the lower portion of the crucible in a cold-crucible-induction melter in order to facilitate discharging molten material therefrom. However, conventional approaches for preferential heating of the bottom portion of the crucible include movement of either the induction coil disposed surrounding the crucible or the crucible disposed therein.
U.S. Pat. No. 6,058,741 to Sobolev et al. discloses a movable inductor for use in vitrification systems.
U.S. Pat. No. 5,940,427 to Hürtgen et al. discloses a coreless induction furnace having two induction coils surrounding a crucible wherein the power may be distributed between each induction coil and that an on-off switch may be connected in series with one or more of the coils.
Accordingly, there exists a need for an improved apparatus and method for controlling the temperature distribution within a cold-crucible-induction melter.
In view of the foregoing problems and shortcomings with existing cold-crucible-induction melter processing materials and systems, it would be an advancement in the art to provide improved bottom drain assemblies, lids, and control methods for cold-crucible-induction melters used for vitrification processing.